1. Field of the Invention
The present invention is related to cross country skis and, especially, to those that are used for practicing the traditional step, called alternating step, as opposed to the sliding step.
2. Description of Background and Relevant Information
A cross country ski generally comprises an arched central portion when at rest, with an upward convexity, demarcated by two lines of contact with the ground, front and rear, respectively, separating this central portion respectively from the spatula and the heel of the ski. The support zone of the foot of the skier is located at a medium position with respect to this central part, at least approximately.
While practicing the alternating step, this support of the foot manifests itself between the sole of the ski and the ground, mainly at two very restricted, and localized points or zones.
During impulsion for forward advance, the foot of the skier takes support at the level of the metatarsal-phalanges joint, from which the position of the support point on the ski or the support point of the impulsion is defined. It is at the level of this point that maximum pressure is exercised in order to flatten the ski by elasticity against the slight bend of the central portion. In the sliding phase or advance of the ski, it is by the heel that the foot provides support on the ski. Due to the original camber of the ski, and the relative position of the foot with respect to it, this support of the heel leaves a residual arched zone in front of its application point, which thus corresponds to zero pressure of the ski on the ground, and which is commonly called "waxing chamber". Indeed, it is in this zone that it is wisest to apply to the sole of the ski, a retention wax or any other means resisting backward movement, such as scales, seal skin, anti-backward movement chemical coating, etc . . . This retention wax, or any other means, is only effective when in contact with the ground, that is, during the impulsion phase, stopping the ski from sliding back by the positive reaction that the ground can then exercise on the ski.
This distribution along the sole of the ski is of considerable importance because it conditions the efficiency of the impulsion transmitted by the metatarsal-phalanges support in the impulsion or impulse phase and the quality of the sliding during support of the heel.
For a traditional ski, basically adapted to alternating steps, one notes, as is illustrated in FIG. 1, that this pressure distribution along the ski is not ideal. This diagram represents sequenced contact pressure, in accordance with the position on the ski in abssicus, between spatula S and the heel of the ski Ts, respectively in the impulsion phase, in dotted and dashed lines, and in the sliding phase, in non-broken lines. The support points in impulsion and of the heel are respectively referenced by Pi and T. One notes that if, in the sliding or heel support phase, the waxing chamber 8 or the zero pressure zone does in fact exist, during the impulsion phase (dotted and dashed lines), the pressure, which is maximum at the level of impulsion point Pi, is exercised on either side of this point such that it is very diffused and wide on the one hand, and has relatively little intensity in the zone of the waxing chamber 8 on the other hand, substantially hindering the efficiency of impulsion in this way.